Karl Marx was a philosopher and political economist who adapted classical economic theory to critique capitalism. He argued that markets alienate individuals from their true selves, and that capitalists extract surplus value from human labor. His ideas have inspired many to advocate for revolutionary political change aimed at upsetting capitalism and reducing the gap in wealth dividing the rich from the poor. His writings include Das Capital (1867), Economic and Philosophical Manuscripts of 1844 (1932), and Manifesto of the Communist Party (with Friedrich Engels) (1848).
Quotes included are two from Capital (one from vol. 3, not that good and one from vol. 1, OK), one from the Manifesto and another from the Contribution (which of course I like :).